Coherent optical detection in optical communication systems provides a receiver sensitivity greatly enhanced with respect to direct detection (DD) schemes.
In coherent receivers the optical signal interferes with a local oscillator (which is locked to the signal both in polarization and in phase or frequency) and the beating term, which contains the information of the optical signal, is then directly detected by a photodiode. The local oscillator has an optical power greatly higher than the power of the optical signal and it acts as an amplifier of the beating term so that the thermal noise of the photodetection electronic circuit becomes negligible and the shot-noise quantum limit may be approached. Examples of coherent optical detection schemes are described in article A. W. Davis et al., J. L. T. Vol. LT-5, No.4, April 1987, pp.561-572 (hereinafter referred to as ‘Davis et al’), which is herein incorporated by reference.
Above cited article Davis et al discloses a (binary) differential phase shift keying (DPSK) modulation format received coherently. In binary DPSK, hereinafter called simply DPSK, the electrical binary data signal is differentially encoded (‘pre-coding’) before modulating the optical signal, so that one of the two symbols of the data signal, e.g. a “one”, in the optical DPSK signal corresponds to a change of the optical phase by 180° between two successive bits, while the other symbol corresponds to a succession of two bits in phase.
One advantage of the coherent reception is the capacity to obtain an electrical signal containing information on both amplitude and phase of the optical field. In this respect, from a mathematical point of view the coherent optical detection acts on the field of the optical signal as a linear operator, at this regard similarly to the nature of the chromatic dispersion accumulated by the optical signal during propagation along the optical link. Thus, in principle it is possible to wholly compensate for any accumulated dispersion (e.g. both chromatic and polarization mode dispersion) by suitably equalizing the coherently received electrical signal.
An example of electric dispersion compensation is described in the article by J. H. Winters, J. L. T., vol. 8, pp.1487-1491 (1990).